Principal and Identity Objects
Managed code can discover the identity or the role of a principal through a IPrincipal object, which contains a reference to an IIdentity object. It might be helpful to compare identity and principal objects to familiar concepts like user and group accounts. In most network environments, user accounts represent people or programs, while group accounts represent certain categories of users and the rights they possess. Similarly, .NET Framework identity objects represent users, while roles represent memberships and security contexts. In the .NET Framework, the principal object encapsulates both an identity object and a role. .NET Framework applications grant rights to the principal based on its identity or, more commonly, its role membership.
The identity object encapsulates information about the user or entity being validated. At their most basic level, identity objects contain a name and an authentication type. The name can either be a user's name or the name of a Windows account, while the authentication type can be either a supported logon protocol, such as Kerberos V5, or a custom value. The .NET Framework defines a GenericIdentity object that can be used for most custom logon scenarios and a more specialized WindowsIdentity object that can be used when you want your application to rely on Windows authentication. Additionally, you can define your own identity class that encapsulates custom user information.
The IIdentity interface defines properties for accessing a name and an authentication type, such as Kerberos V5 or NTLM. All Identity classes implement the IIdentity interface. There is no required relationship between an Identity object and the Windows NT process token under which a thread is currently executing. However, if the Identity object is a WindowsIdentity object, the identity is assumed to represent a Windows NT security token.
The principal object represents the security context under which code is running. Applications that implement role-based security grant rights based on the role associated with a principal object. Similar to identity objects, the .NET Framework provides a GenericPrincipal object and a WindowsPrincipal object. You can also define your own custom principal classes.
The IPrincipal interface defines a property for accessing an associated Identity object as well as a method for determining whether the user identified by the Principal object is a member of a given role. All Principal classes implement the IPrincipal interface as well as any additional properties and methods that are necessary. For example, the common language runtime provides the WindowsPrincipal class, which implements additional functionality for mapping Windows NT or Windows 2000 group membership to roles.
A Principal object is bound to a call context (CallContext) object within an application domain (AppDomain). A default call context is always created with each new AppDomain, so there is always a call context available to accept the Principal object. When a new thread is created, a CallContext object is also created for the thread. The Principal object reference is automatically copied from the creating thread to the new thread's CallContext. If the runtime cannot determine which Principal object belongs to the creator of the thread, it follows the default policy for Principal and Identity object creation.
A configurable application domain-specific policy defines the rules for deciding what type of Principal object to associate with a new application domain. Where security policy permits, the runtime can create Principal and Identity objects that reflect the operating system token associated with the current thread of execution. By default, the runtime uses Principal and Identity objects that represent unauthenticated users. The runtime does not create these default Principal and Identity objects until the code attempts to access them.
Trusted code that creates an application domain can set the application domain policy that controls construction of the default Principal and Identity objects. This application domain-specific policy applies to all execution threads in that application domain. An unmanaged, trusted host inherently has the ability to set this policy, but managed code that sets this policy must have the System.Security.Permissions.SecurityPermission for controlling domain policy.
When transmitting a Principal object across application domains but within the same process (and therefore on the same computer), the remoting infrastructure copies a reference to the Principal object associated with the caller's context to the callee's context.